For a conventional valve device 100, there is known a throttle valve device including a body 101 that defines an intake passage, and a butterfly valve 102 that is supported rotatably in the body 101 and is driven by an actuator (not shown), as illustrated in FIG. 16.
The body 101 includes a flow passage forming chamber 103 that is a part of the intake passage, and a shaft hole 105 which opens into the flow passage forming chamber 103 and through which a rotation shaft 104 of the butterfly valve 102 passes. Bearings 106 that support the rotation shaft 104 are arranged between the shaft hole 105 and the rotation shaft 104.
In this valve device 100, a clearance 110 may be formed between the butterfly valve 102 and the bearing 106, which are opposed to each other in the axial direction of the rotation shaft 104 (left-right direction in FIG. 16). In the case of this structure, the clearance 110 is allowed for the thermal expansion of the butterfly valve 102 to be able to avoid a biting phenomenon (phenomenon of the butterfly valve 102 crimped against the bearing 106 to be incapable of rotating).
However, the clearance 110 normally serves as a flow passage that causes the leakage of intake air from the upstream side to the downstream side of the butterfly valve 102 (in the thickness direction of a plane of paper of FIG. 16) when the butterfly valve 102 is fully closed. Thus, the flow rate of leaking intake air (hereinafter referred to as an intake air leak flow rate) increases.
As the measures against this, Patent Documents 1, 2 disclose the structure of the butterfly valve and the bearing which are in contact. Specifically, in this structure, the surface of the bearing on the flow passage forming chamber-side is exposed to the flow passage forming chamber so that the butterfly valve can be in sliding contact with the bearing. Although this reduces the intake air leak flow rate, this structure may render the butterfly valve incapable of rotating due to the occurrence of the biting phenomenon when the butterfly valve is thermally expanded.
Thus, there is a demand for the structure that can reduce the leakage of fluid from the upstream side to the downstream side of the butterfly valve when fully closed and that can avoid the phenomenon of the butterfly valve incapable of rotating when thermally expanded.